Externally-reset tachycardia control pacer

ABSTRACT

An improved pacer for controlling tachycardia. Placing a magnet over the patient&#39;s chest results in the generation of two pulses, the time separation between which is an indication of the battery potential. Application of the magnet in this way also holds the device off, after the two pulses are generated, so that if the device is causing him discomfort the patient can temporarily disable it until the physician can program it off. Application of a magnet to the patient&#39;s chest, or programming of the device, also resets two start-of-scan time intervals to the values programmed by the physician. The next scanning begins with the programmed time values. In this way the physician, after inducing tachycardia, can verify the programmed time parameters rapidly by observing the patient&#39;s ECG waveform, without having to wait several minutes until scanning from previously retained successful values would otherwise progress to the programmed values.

This invention relates to tachycardia control pacers, and moreparticularly to such pacers which can be externally controlled (otherthan during programming).

Tachycardia is a condition in which the heart beats very rapidly,typically, above 150 beats per minute. There are several differentpacing modalities which have been suggested for termination oftachycardia. The underlying principle in all of them is that if a pacerstimulates the heart at least once shortly after a heartbeat, before thenext naturally occurring heartbeat at the rapid rate, the heart maysuccessfully revert to sinus rhythm. Tachycardia is often the result ofelectrical feedback within the heart; a natural beat results in thefeedback of an electrical stimulus which prematurely triggers anotherbeat. By interposing a stimulated heartbeat, the stability of thefeedback look is disrupted. As with conventional heart pacers, theelectrodes of a tachycardia control pacer may be atrially-coupled orventricularly-coupled. Although the detection of atrial beats and atrialstimulation are preferred, ventricular beat detection and pacing mayalso be employed.

The difficulty in tachycardia control is that there is usually no way ofknowing exactly when a stimulating pulse should be applied. It must beapplied shortly after a heartbeat and prior to the time when the nextpremature beat would otherwise occur, but there is usually only a shortperiod of time somewhere between successive beats during which thegeneration of a stimulating pulse will successfully terminatetachycardia. In Spurrell-Allen-Kenny U.S. Pat. No. 3,942,534, entitled"Device for Terminating Tachycardia" and issued on Mar. 9, 1976(corresponding to British Pat. No. 1,493,353 dated Nov. 30, 1977), thereis disclosed a pacer which, following detection of tachycardia,generates a stimulus after a delay interval. If that stimulus is notsuccessful in terminating the condition, then another stimulus isgenerated after another premature heartbeat following a slightlydifferent delay. The device constantly adjusts the delay interval by"scanning" through a predetermined delay range. Stimulation ceases assoon as the heart is restored to sinus rhythm. If successful reversionis not achieved during one complete scan, then the cycle is repeated.

The above-identified Spurrell et al patent further teaches thegeneration of a second stimulus following the first, both stimulioccurring within the tachycardia cycle, i.e., before the next naturallyoccurring rapid beat. It has actually been found that the secondstimulus may be more effective than the first. As used herein, the timeperiod between a heartbeat and the first stimulus is referred to as the"initial delay", and the time period between the first stimulus and thesecond stimulus is referred to as the "coupled interval". In theSpurrell et al device, although the coupled interval may be set by thephysician, it is fixed once it is set; the second stimulus always occursa predetermined time after the first stimulus, no matter when the firststimulus occurs after the last heartbeat.

In Spurrell-Nappholz-Swift application Ser. No. 245,215, filed on evendate herewith and entitled "Two-Pulse Tachycardia Control Pacer"(corresponding to British application Ser. No. 8,010,013, filed on Mar.25, 1980 and entitled "Heart Stimulating Device"), which applicationcontains the same detailed disclosure contained herein, there is claimeda tachycardia control pacer in which the time intervals which aresuccessful in terminating tachycardia are permanently stored so that, nomatter when the next tachycardia episode occurs, the scanning beginswith the most recent successful time parameters. While this is noguarantee that the first pair of stimuli will necessarily result insuccessful tachycardia termination, on average it takes many fewerstimuli to achieve successful reversion because the scanning alwaysbeings with the last successful time parameters. Also, there is claimedthe scanning of the coupled interval, as well as the scanning of theinitial delay. By registering the successful coupled interval as well asthe successful initial delay and using the two retained parameter valuesfirst when the next tachycardia episode is detected, there is a muchgreater likelihood of success with the first pair of stimuli the nexttime they are required.

The tachycardia control pacer claimed in the Spurrell-Nappholz-Swiftapplication is programmable; using conventional heart pacer programmingtechniques, the physician can program several parameters. Among theseare the initial delay and coupled interval values at extreme ends of therespective scans. Since the scanning of both parameters occurs infifteen 6-millisecond steps, programming of the two extreme valuesdetermines the entire range through which each scan takes place. Theselection of parameter values is facilitated by externally controllingthe pacer to actually induce tachycardia. By allowing the physician toinduce the condition and then to monitor the patient to see the effectof different parameter values, optimum values can be programmed for eachindividual patient. Thus the tachycardia control pacer can actually beoperated in a mode, under external control, which induces tachycardia,following which the physician can observe whether the devicesuccessfully terminates the condition rapidly with the programmedparameter values.

A conventional heart pacer is often designed so that placing a magnetover the chest of a patient, in the vicinity of the pacer, controlscontinuous pacing at a rate which reflects the battery potential; thisallows the remaining life of the pacer to be ascertained. This is notfeasible in the case of a tachycardia control pacer, however, becausethe device does not function to generate continuous pulses. It istherefore an object of our invention to provide a mechanism for atachycardia control pacer which allows determination of the batterypotential.

It has also been found that at times a patient may experience discomfortfrom the operation of a tachycardia control pacer. It is another objectof our invention to provide a simple mechanism by which a patient cancontrol temporary turnoff of such a pacer, until the physician canprogram it off.

It sometimes happens that a physician does not know the programmedvalues for initial delay and coupled interval. This is especially trueif a patient visits a new physician. The programmed values can bedetermined if the physician, after inducing tachycardia, observes thepatient's ECG waveform and waits until the scanning sequence reaches thepoint at which the maximum time values are employed during a cycle. Thiscan often take several minutes. It is therefore another object of ourinvention to provide a mechanism by which a physician can rapidlydetermine the programmed values.

All of the aforesaid objectives are accomplished by operating a reedswitch (the same reed switch which is operated by the externalprogrammer) while a magnet is placed over the chest of the patient inthe vicinity of the pacer. Operation of the switch turns off the pacerso that the patient can temporarily hold the device off while he travelsto his physician in order to have the physician program the device off.But before the device turns off it generates two pulses, the timeseparation between which is a measure of the battery potential. Thus thephysician can use a magnet to "turn off" the device for the purpose ofgenerating these two pulses. If he observes the ECG waveform of thepatient and times the interval between the two pulses which are actuallygenerated before the device is turned off, he will be able to ascertainthe battery potential.

The same mechanism which responds to the external magnet (or theexternal programmer) also resets the scanning cycling so that the nextcycle utilizes the programmed values of initial delay and coupledinterval. Consequently, by resetting the scanning cycling (either byusing an external magnet or an external programmer), the physician cancontrol the scanning to begin with the programmed values. Immediatelyfollowing the inducing of tachycardia, the physician can observe theprogrammed values on the patient's ECG waveform.

Further objects, features and advantages of our invention will becomeapparent upon consideration of the following detailed description inconjunction with the drawing, in which:

FIGS. 1 and 2, arranged as shown in FIG. 2A, depict the illustrativeembodiment of our invention;

FIGS. 3 and 4, arranged as shown in FIG. 4A, depict the circuitrycontained in chip IC4 of FIG. 2;

FIGS. 5-8, arranged as shown in FIG. 8A, depict the circuitry containedin chip IC3 of FIG. 2.

FIG. 9 depicts the details of monostable multivibrator MN1 which isshown only in block form on FIG. 5; and

FIGS. 10-14 depict timing waveforms which will facilitate anunderstanding of the pacer operation.

TIMING WAVEFORMS: FIGS. 10-14

For an understanding of the invention, it is better to begin with whatthe pacer does rather than how it does it. For this reason, the timingwaveforms of FIGS. 10-14 will be described first.

The first waveform in FIG. 10 depicts at the left five heartbeats. Inthe illustrative embodiment of the invention, a tachycardia episode isdetected (referred to as a tachycardia "confirmation") if each of foursuccessive heartbeats occurs within a predetermined time intervalfollowing the preceding respective heartbeat. (This time interval may beprogrammed by the physician, as will be described below.) It is presumedthat the first beat in FIG. 10 occurred after the preceding heartbeat(not shown) by more than the predetermined interval, but that the fourinterbeat intervals following this heartbeat were all too short. Upondetection of the fifth heartbeat in the sequence, a tachycardia episodeis assumed to be in progress. The upper waveform shows a single stimulusst being delivered t seconds after the last heartbeat in the sequence, tseconds corresponding to the initial delay. The upper waveform depicts areversion to sinus rhythm as a result of the single stimulus, with thesucceeding heartbeats being separated by more than the predeterminedinterval which controls tachycardia confirmation. The second waveform inFIG. 10 depicts the application of the same single stimulus, which inthis case does not result in tachycardia reversion; it is seen that thesucceeding heartbeats are still too rapid.

The last waveform in FIG. 10 depicts (not to scale) the application of asingle stimulus st after the fifth hearbeat which comprises atachycardia confirmation cycle, following an initial delay which isshorter than t milliseconds by Δ milliseconds. In the illustrativeembodiment of the invention, the value of Δ is six milliseconds. Thethird waveform simply illustrates what is meant herein by decrementingthe initial delay by the decrement value. The initial delay is reducedby six milliseconds from cycle to cycle, as will be described below. Inthe last case of FIG. 10, the stimulus which followed the shorterinitial delay is shown as having been successful in terminatingtachycardia.

While FIG. 10 depicts a single stimulus as being successful orunsuccessful in terminating tachycardia, it does not show how theinitial delay is varied over its entire range, nor does it depict asecond stimulus at all. The actual sequencing of the pacer of ourinvention will be described with reference to FIGS. 12-14. But beforeproceeding to the actual sequencing, it will be helpful to understandthe mechanism by which a stimulus can terminate tachycardia. The topwaveform in FIG. 11 shows two heartbeats in a tachycardia cycle (thescale being expanded relative to that of FIG. 10). The pacer of ourinvention controls a "scan window" between the two heartbeats. Thewindow is 90 milliseconds in duration. It is assumed that a stimulussomewhere within the scan window will be effective in terminatingtachycardia; the exact position within the window where the occurrenceof a stimulus will be successful may be anywhere within a small rangereferred to as the "region of susceptibility". If the region ofsusceptibility is that shown in the upper waveform of FIG. 11, then astimulating pulse which occurs in this region will be successful interminating tachycardia. On the other hand, if the region ofsusceptibility is toward the beginning of the scan window, as shown inthe middle waveform of FIG. 11, then it is only a pulse within thisregion--closer in time to the last heartbeat--which will result insuccess.

The physician programs the pacer with an initial delay value which isthe time between the last heartbeat in the tachycardia confirmationcycle and the right end of the scan window (scan window No. 1 in the twoupper waveforms of FIG. 11). It is assumed that the region ofsusceptibility is somewhere within the scan window, although exactlywhere is not known; that is why the first stimulus is generated after aninitial delay which is different from cycle to cycle.

The bottom waveform in FIG. 11 depicts the case in which the region ofsusceptibility is not within scan window No. 1. In such a case, thegeneration of a stimulus at no position within this scan window willresult in reversion. It is assumed, therefore, that there is some otherscan window which contains a region of susceptibility, this window beingshown as scan window No. 2. The physician can program the device toplace the 90-millisecond scan window at a selected position after thelast heartbeat which confirms tachycardia. Thereafter, the pacerautomatically generates stimuli in succeeding cycles (corresponding to asingle cycle as shown in FIG. 10) in different potential regions ofsusceptibility; the initial delay is always decremented by 6milliseconds from cycle to cycle (except in going from the minimuminitial delay to the maximum, at the start of a new scan), but thephysician does have control over the scan window.

This is shown in greater detail in FIG. 12, which depicts the cycling ofthe pacer but where a second stimulus is still not generated during eachcycle. Following the confirmation of tachycardia, only a single stimulusis generated. The pacer then assumes that the heart is beating normallyand determines that another stimulus is required only if tachycardia isonce again confirmed by counting four rapid heartbeats, following whichanother single stimulus is generated during the next cycle.

Waveform (a) in FIG. 12 depicts a single cycle. The interval trepresents the initial delay programmed by the physician; in theillustrative embodiment of the invention the initial delay is themaximum delay during any scan. Thus the first stimulus which isgenerated, during the first cycle, is shown by the symbol st and occurst milliseconds after the last heartbeat in the confirmation sequence.

Waveform (b) in FIG. 12 depicts the occurrence of the stimulus duringthe next overall cycle. It must be borne in mind that the next cycleoccurs only after tachycardia is confirmed all over again without areversion to sinus rhythm. In this case, the initial delay is (t-6)milliseconds, and the stimulus depicted in waveform (b) represents thestimulus which occurs following the first 6-millisecond decrement. Thestimulus occurs, of course, within the 90 millisecond scan window.

Waveform (c) shows the occurrence of the stimulus after the initialdelay has been decremented n times. The stimulus still occurs within thescan window, but it now sooner follows the last heartbeat in theconfirmation sequence.

Waveform (d) depicts the sixteenth stimulus in an overall scan, afterthe fifteenth decrement. The stimulus occurs at the start of the scanwindow. If this stimulus is not successful in terminating tachycardia,then following the next confirmation a stimulus is generated after tseconds have elapsed in the tachycardia cycle; the scanning begins allover again with the maximum (programmed) initial delay, assuming thatthere has been no reversion to sinus rhythm.

In the illustrative embodiment of the invention, the second stimulus maybe omitted altogether, as depicted in FIG. 12. But if it is used, it canbe generated after the first stimulus by either a fixed coupled interval(which can be programmed), or after a variable, scanned coupledinterval. FIG. 13 depicts the case in which the coupled interval isfixed at T milliseconds and does not vary. Waveform (a) shows the twotime interval parameters programmed by the physician, t milliseconds andT milliseconds. The first stimulus in a new scan occurs t millisecondsafter tachycardia confirmation, and the second stimulus occurs Tmilliseconds later. Waveform (b) depicts the scanning of the initialdelay, with the initial delay having been decremented n times, or 6 nmilliseconds. It will be noted that the second stimulus still occurs Tmilliseconds after the first. Finally, waveform (c) shows the firststimulus occurring after the shortest initial delay, with the secondstimulus still occurring after the fixed coupled interval of Tmilliseconds. It is thus apparent that in the arrangement depicted inFIG. 13, stimuli are generated within the two scan windows depicted bythe letters A and B. The first stimulus always occurs within the windowdepicted by the letter A, and the second always occurs within the windowdepicted by the letter B; no stimuli are generated followingconfirmation within the region between windows A and B.

However, if the coupled interval is scanned, i.e., the time betweenstimuli is varied, there may be no gap between windows A and B duringwhich a stimulus is not generated. Once again, in FIG. 14 the letter trepresents the programmed initial delay, and the letter T represents theprogrammed coupled interval. At the start of a scanning cycle, themaximum time intervals are both utilized. Thus the first stimulus occurst milliseconds after tachycardia is confirmed, and the second stimulusoccurs T milliseconds after the first stimulus. This is shown inwaveform (a). The letter A still represents a 90-millisecond interval.For reasons which will become apparent, however, the letter B nowrepresents a longer time "space" in which the second stimulus can occur.

For any given value of coupled interval, the first stimulus is scannedthrough its respective window. It is only after a complete scan throughthe 90-millisecond initial delay window that the coupled interval isdecremented by 6 milliseconds, and that the new value for the coupledinterval is used while another complete scan of initial delay takesplace. Waveform (b) shows when the first and second stimuli aregenerated during the second cycle. The first stimulus is generated 6milliseconds earlier than was the first stimulus during the precedingcycle. The second stimulus is still generated T milliseconds after thefirst stimulus. Of course, since the first stimulus now occurs 6milliseconds earlier, so does the second stimulus. Waveform (c) showsthe first and second stimuli which are generated at the end of the scanof the initial delay. It will be noted that the operation described thusfar is the same as that depicted in FIG. 13, since the coupled intervalis fixed at T milliseconds during the first scan of the initial delay.

Waveform (d) depicts the two pulses which are generated at the start ofthe next scan of the initial delay. As in waveform (a), the firststimulus is generated t milliseconds after tachycardia confirmation. Butthe coupled interval is now 6 milliseconds shorter than it was duringthe first scan of the initial delay. The scanning of the initial delaynow takes place in the same way, with the new value of T-6 millisecondsbeing used for the coupled interval each time. If reversion to sinusrhythm is not achieved, then the coupled interval is decremented onceagain by 6 milliseconds, the new coupled interval being used for anothercomplete scan of the initial delay.

Waveform (e) depicts the values of the initial delay and the coupledinterval at the start of the very last scan of the initial delay. It isassumed that the coupled interval has been decremented down to itslowest value of T-90 milliseconds. Thus at the start of the last scan ofthe initial delay, the first stimulus is generated t milliseconds aftertachycardia confirmation, and the second stimulus is generated T-90milliseconds later. The initial delay is then decremented by 6milliseconds during succeeding cycles, while the coupled intervalremains fixed at T-90 milliseconds. Waveform (f) depicts the end of theoverall scanning sequence with both the initial delay and the coupledinterval being at their lowest values. If success is not achieved, thenthe system starts all over again with the two programmed values, asshown in waveform (g).

It will be observed that the second stimulus in waveform (f) isgenerated at a time corresponding to the left-most end of time "space" Bin FIG. 14. Thus it is apparent that with a maximally flexible systemsuch as that illustrated by FIG. 14, the first stimulus ranges withintime "space" or window A, while the second stimulus ranges within a time"space" B, which may or may not overlap time "space" A.

In general, the initial delay should be scanned over at least a60-millisecond range, and its maximum value should be programmable overat least a range of 150 milliseconds. The coupled interval shouldsimilarly be scanned over at least a 60-millisecond range, but itsmaximum value should be programmable over at least a range of 200milliseconds.

As will be described in detail below, the actual cycling is slightlydifferent from that depicted in FIG. 14 for two reasons. First, thecycling does not necessarily begin with the two programmed values of tand T. Instead, the two values which were used during the lastsuccessful tachycardia termination are the first to be tried, with thecycling then proceeding with the decrementing of the initial delay valueas just described. Second, for any value of coupled interval T, thereare actually two scans of the initial delay. The cycling begins with theretained t and T values. After the initial delay has been decrementeddown to its lowest value, the coupled interval is not decremented asimplied above. Instead, the programmed initial delay value is used tostart another scan of the initial delay, and the previously successfulcoupled interval value is used during the new scan of initial delay.This is because the first scan of the initial delay does not begin withthe programmed initial delay value, but rather with the retained initialdelay value. Thus there is only a partial scan of the initial delay and,if it is not successful, it is preferred to provide a complete scan ofthe initial delay while using the previously successful coupled intervalvalue, just in case the retained coupled interval value will besuccessful with an initial delay value which is longer than thepreviously successful initial delay value. It is only after the secondinitial delay scan that the coupled interval is decremented. It is notnecessary thereafter to provide two scans of the initial delay for eachnew value of coupled interval. Nevertheless, this is what is actuallydone in the illustrative embodiment of the invention in order tosimplify the circuitry. Thus in actual practice what happens is thateach time the coupled interval is decremented, there are two scans ofthe initial delay, and only after the second scan is the coupledinterval decremented again by 6 milliseconds.

Monostable Multivibrator: FIG. 9

Chip IC3 on FIG. 2 is the most complex of the five chips IC1-IC5 shownon FIGS. 1 and 2. The details of chip IC3 are depicted in FIGS. 5-8. Oneof the elements on the chip is multivibrator MN1 on FIG. 5. Theoperation of the multivibrator, from a system point of view, is verystraight-forward. It is a re-triggerable device which generates apositive pulse at its Q output each time a trigger is received at the Ainput. If another trigger is received before the multivibrator has timedout, the Q output remains high for another timing period. Themultivibrator is used to confirm tachycardia, and to understand thesystem operation the details of the multi-vibrator are unimportant. Inorder that the description of the system not be complicated by thedetails of the multivibrator operation, it would be best to consider themultivibrator at this point so that in the system description thedetailed operation of the element can be ignored.

In FIG. 5, multivibrator MN1 is shown as having five inputs/outputs. TheQ output is normally low in potential and Q output is normally high. Apositive potential at the reset (R) input resets the multivibrator inthis state. Upon receipt of a positive trigger pulse at the trigger (A)input, however, the Q output goes high and the Q output goes low. Theduration of the pulse is controlled by various components connected topins 1 and 2 on chip IC3. (The two pins are shorted to each other.)Referring to FIG. 2, it will be noted that pins 1 and 2 on chip IC3 areconnected to capacitor C9, the other end of which is grounded through200-ohm resistor R29. (The junction the resistor and capacitor alsoserves as the V_(SS) connection to chip IC3 at pins 21,22, as shown onFIG. 6.) Pins 1 and 2 on chip IC3 are also connected to a resistorchain, the first resistor of which is R21, as shown on FIG. 2. As willbe described below, some of the resistors in the chain are shorteddepending upon how the pacer has been programmed. But the totalimpedance determines the programmed "tachy rate", that is, the minimuminter-beat interval which, if exceeded, will abort a tachycardiaconfirmation cycle.

The multivibrator is shown in detail on FIG. 9. The reset and triggerinputs are shown on the left of the drawing, and the Q and Q inputs areshown in the upper right corner. Pins 1 and 2 of chip IC3 are shownconnected on FIG. 9 to capacitor C9, just as the pins are shownconnected on FIG. 2. However, instead of showing the complete resistorchain on FIG. 9, as it is shown on FIG. 2, the resistor chain is simplyshown by a single impedance designated R.

On FIG. 9, V_(DD) represents the battery potential, nominally 2.8 volts.Conventional symbols are employed to represent CMOS P-channel andN-channel enhancement-mode transistors, with designations such as P/2 or2P referring to relative "on" impedances, that is, a 2P device conductstwice as much current as a P device for the same gate-source bias. Theother devices shown comprise standard CMOS gates; symbols such as 3Xadjacent an inverter refer to the fact that three standard inverters areconnected in parallel.

In the absence of any trigger inputs, both of transistors 100 and 102are off. Capacitor C9 charges through the resistor chain symbolized bythe single resistor R from the positive supply, and pins 1 and 2 are ata high potential. The various devices which comprise the "inverter"function as a comparator. The 6 P-channel devices connected in seriesderive a threshold voltage which is equal to about half of the supplyvoltage. This threshold voltage is compared with the potential at pins 1and 2. As long as the potential at pins 1 and 2 exceeds the thresholdvoltage, the Q output is low (thus the terminology "inverter", althoughthe circuit also functions as a comparator). It is only when thecapacitor voltage is less than the threshold voltage that the Q outputis high and the Q output is low.

As will be described below, the trigger inputs represent heartbeats. Thetrigger input is normally low in potential. Gates 104,106 comprise afirst latch, and gates 108,110 comprise a second latch. After themultivibrator times out, the output of latch 1 (output of gate 106) ishigh in potential and the output of latch 2 (output of gate 108) is lowin potential. Latch 1 is considered to be reset, and latch 2 isconsidered to be set.

When a heartbeat is detected, the positive pulse at the trigger inputcauses latch 1 to set and the output of gate 106 goes low. Because bothlatch outputs are now low, and the two outputs are connected to theinputs of gate 112, its output now goes high. After being invertedtwice, the high output of gate 112 causes transistor 102 to turn on.This has the effect of causing capacitor C9 to rapidly discharge throughthe device; the Q output now goes high and the Q output goes low.

The elements which comprise the "buffer" are a form of comparator, andthey serve to detect when the capacitor voltage drops to about 100millivolts. As long as the capacitor voltage exceeds 100 millivolts, theinput to inverter 114 is low in potential. One input of gate 115 is thushigh and the output of inverter 118 is similarly high; it is this highpotential which holds latch 2 set with the output of gate 108 being low,as originally assumed. The second input of gate 115 is connected to theoutput of gate 106 which is also originally high. Thus in the absence ofany heartbeats, both inputs to gate 115 are high, latch 2 remains set,and latch 1 remains reset. Even after a heartbeat is detected and theoutput of gate 106 goes low, latch 2 remains set because the output ofinverter 114 is still high.

But as soon as capacitor C9 discharges through transistor 102 to thepoint at which its potential drops to 100 millivolts, the output ofinverter 114 goes low. Since latch 1 is now set with the output of gate106 also being low, the output of inverter 118 goes low. This causeslatch 2 to reset, with the output of gate 108 going high. The highpotential at the output of gate 108 causes the output of gate 112 to golow, and immediately causes transistor 102 to turn off. The highpotential at the output of gate 108 is also inverted by inverter 120 toreset latch 1, with the output of gate 106 going high once again. Thehigh potential at the output of gate 106 now sets latch 2 once again (oras soon as the trigger pulse terminates), since the output of gate 106is connected to an input of gate 115. Consequently, latch 2 is set onceagain in its quiescent condition just as latch 1 is reset in itsquiescent condition. Although the output of gate 108 no longer holdstransistor 102 off, it is now the high output of gate 106 which holdsthe transistor off. The capacitor now starts to charge once againthrough the resistor chain.

It is thus apparent that following each heartbeat and the discharge ofcapacitor C9, the Q output of the multivibrator goes high and the Qoutput goes low. As soon as the capacitor charges to the threshold levelof the inverter, in a time dependent upon the magnitude of impedance R,the output pulse terminates and the Q output goes low once again withthe Q output going high. But if another heartbeat is detected before thecapacitor can charge to the threshold level, the charging cycle beginsall over again as soon as the capacitor discharges through transistor102, and the Q output remains high. Thus the Q output remains high assuccessive heartbeats are detected, without going low between them, onlyif the heartbeats are detected at a rate fast enough to preventcapacitor C9 from charging to the threshold voltage of the "inverter".The multivibrator is re-triggerable in the sense that each trigger inputextends the output pulse (positive potential at the Q output) foranother time-out interval. As will be described below, this is the basicmechanism for detecting a tachycardia episode--as long as fourheartbeats are detected after an initial triggering of themultivibrator, without the Q output of the multi-vibrator going low,then it is assumed that a tachycardia episode has been detected. If anypair of successive heartbeats are separated by a time interval whichexceeds that required for capacitor C9 to charge to the threshold level,then the Q output of the multivibrator goes low. As will be describedbelow, this aborts the tachycardia confirmation counting cycle. Byprogramming the value R, the physician can determine the heartbeat ratewhich if exceeded will result in tachycardia detection. The effectiverates which the physician can program vary between 130 and 225 beats perminute. (As will be described below, the physician can also "fool" thepacer by programming it to a "tachy rate" of only 40 beats per minute;this results in the pacer treating normal beats as a tachycardiaepisode, the pacer automatically generates "premature" stimuli in aneffort to terminate non-existing tachycardia and this may actuallyinduce real tachycardias. By then reprogramming the pacer with a normal"tachy rate", the physician can check whether the programmed timeparameters are effective in terminating tachycardia.)

The reset input on FIG. 9 is normally low in potential. The high outputof inverter 122 holds transistor 100 off and also holds one input ofgate 108 high so that latch 2 can operate as described above. But whenthe reset input is high, as will be described below, the output ofinverter 122 is low in potential and transistor 100 turns on. This hasthe effect of rapidly charging capacitor C9 through transistor 100 fromthe V_(DD) supply, and the Q output of the multivibrator remains low (asthough heartbeats were not being detected at all) for as long as thereset input is high. The low input now applied by inverter 122 to oneinput of gate 108 causes the output of latch 2 to be forced high, andthis in turn holds the output of gate 106 high even if trigger pulsesare received. In this way, transistor 102 remains off independent oftrigger inputs.

The reason for the relatively "complex" multivibrator is that thebattery supply, while nominally 2.8 volts, can drop as low as 2.2 voltswith age. In order that the time-out remain constant independent of thebattery potential, the circuit is designed to provide a threshold whichis equal to about half of the battery supply, no matter what its value.This is the function of the six P-channel transistors in the "inverter"circuit. Since the battery potential determines both the level to whichcapacitor C9 charges and the threshold potential, the time-out isindependent of the precise potential level.

It should be noted that this type of multi-vibrator is standard in theart. In fact, Motorola Inc. markets a component (MC14538) which is justsuch a multivibrator; its time-out period is independent of supplyvoltage. But the circuit of FIG. 9 is preferred because it operates onlow voltages and draws very little current (thereby extending devicelife).

Overview Of The System And General Chip Descriptions

The overall system is shown on FIGS. 1 and 2, and it includes five chipsIC1-IC5. Chips IC1, IC2 and IC5 are standard-type chips used in heartpacers; they will be described below only in terms of their inut andoutput signals, and the functions which they perform. Chips IC3 and IC4are specially-designed chips and they will be described in detail. Thecircuitry included on chip IC4 is shown in FIGS. 3 and 4, and thecircuitry included on chip IC3 is shown in FIGS. 5-8.

Each of the five chips on FIGS. 1 and 2 is designated not only by one ofthe labels IC1-IC5, but also by its chip number, e.g., chip IC5 bearsnumber 1532C. On each of the two sets of chip drawings of FIGS. 3 and 4,and FIGS. 5-8, each pin of the respective chip is labeled not only bynumber, but also by its connection in the overall system. For example,pin 5 of chip IC3 (see FIG. 5) has adjacent to it the designation1400R/15,16. This means that pin 5 of chip IC3 is connected to pins 15and 16 of chip IC2 (1400R). Referring to FIGS. 1 and 2, it will be seenthat pin 5 of chip IC3 is indeed connected to pins 15 and 16 of chipIC2. As another example, pin 21 of chip IC4 (see FIG. 4) bears thedesignation *R22. This means that pin 21 of the chip is connected toresistor *R22, as shown in FIGS. 1 and 2.

In FIGS. 1 and 2, it will be noted that several of the resistors have anasterisk preceding their labels; this symbol identifies a resistor asbeing a high-stability component. Several of the resistors are notprovided with component values, and instead are labeled "SOT". Such adesignation refers to the fact that the value of the respectivecomponent is "selected on test", i.e., a component value is selectedwhich provides proper operation. The component ranges for the resistorsdesignated as SOT are as follows:

R13: 8.06-11.5M

R8: 220-420K

R17: 4.81-8.66M

R18B: 8.2-11.5M

R27: 1.2-2.4M

R15: 3.9-6.8K

It will also be noted that many of the inputs and outputs of the chipson FIGS. 1 and 2 have two pin designations. For example, chip IC2 onFIG. 1 is connected to the positive supply rail via two pins 23,24. Itis standard practice in the pacer art to provide such double connectionsfor increased reliability; even if one pin connection fails, because thetwo pins are internally connected on the chip, the chip still functionsfor its intended purpose as long as the other pin connection remainsintact.

Chip IC1 is a conventional sense amplifier/comparator, and chip IC2 is aconventional timing oscillator/pulse doubler; both chips are standardchips used in the manufacture of heart pacers and are available fromAmalgamated Wireless Microelectronics Pty. Ltd. of Sydney, Australia.Chip IC5, used by Telectronics Pty. Ltd. in its standard line of heartpacers, is a standard-type "program controller" chip; this chip detectsreed closures, as controlled by an external programmer, and setsprogrammable parameters accordingly in the pacer. Techniques forprogramming pacers are standard in the industry, the design of programcontrollers is well known in the art, and there is nothing unique aboutuse of the particular chip No. 1532C insofar as the present invention isconcerned; any conventional programming technique may be employed, aslong as it provides the signals to be described below. Chip IC4 servesprimarily to store programmed values and to control the shorting out ofselected resistors in two resistor chains. Chip IC3 contains most of thelogic which is unique to the present invention.

Programming Of The Pacer: Chips IC4 and IC5, And The Resistor Chains

Before proceeding to a detailed description of the system operation, themanner in which the pacer can be programmed will first be described. Inthis way, it will be understood how the various latches containparameter values when the data stored in these latches are describedbelow as controlling respective functions. The programming isessentially independent of the system operation, and it will beconvenient to describe it first so that the pacing functions can beconsidered below without having to digress for the purpose of describingthe programming.

Chip IC5 on FIG. 1 (1532C) is a conventional-type program controller.The V_(DD) connection to the chip is at pins 23,24. (As shown at thebottom of FIG. 1, the positive supply potential, V+, is derived from a2.8-volt cell, with a filter capacitor C12 connected across it.) Reedswitch RS1 is connected to pins 15,16, with resistor R26 serving as thepull-up for the switch. Under the influence of an external magneticfield, the normally-open reed switch is closed, and a ground potentialis applied to pins 15,16. Resistor R27 and capacitor C11 are the timingcomponents for an internal oscillator on the chip. Incoming reed pulsesmust be properly timed if an incoming programming sequence is to betreated as valid; the internal oscillator on the chip determines whethervalid programming pulses are received. For example, if the reed switchis held closed for a long time period by placing an external magnet overthe chest of the patient, because the resulting pulse at pins 15,16 istoo long relative to the oscillator timing, the reed closure has noeffect on the outputs of chip IC5.

There are six parameters which may be programmed. The first is pulsewidth, i.e., the width of each pulse generated by the pacer. Two bitsare used to represent the pulse width, and there are thus four possiblevalues. The first value is 0--effectively disabling the pacer since nopulses are generated. The three pulse widths which can be controlledwhen the pacer is operative are 0.25, 0.35 and 0.6 milliseconds.

The second parameter is sensitivity. A single bit is used to controlsensitivity of the sense amplifier/comparator chip IC1, as is standardin the pacer art. The two sensitivities are 1 millivolt and 2millivolts.

The third programmed parameter pertains to the second stimulus which isgenerated during each pacing cycle. As depicted in FIG. 12, in somecases it may be desired not to have a second stimulus at all. Asdepicted in FIG. 13, in some cases it may be desired to have a secondstimulus which always follows the first stimulus by a fixed (programmed)coupled interval. Finally, as depicted in FIG. 14, in other cases it maybe desired to have a second stimulus which follows a coupled intervalwhich is scanned (the maximum coupled interval being programmed). Onebit is required to represent whether a second stimulus is generated atall. If it is, another bit represents whether the coupled interval isfixed or scanned.

The fourth parameter is the maximum initial delay value, shown by theletter t on FIGS. 12-14. Scanning of the initial delay begins with thisvalue the first time that the pacer is called upon to terminatetachycardia after the initial programming. (Thereafter, the successfulinitial delay is retained, and subsequent scanning begins with theretained value.) There are 12 maximum initial delay values from whichthe physician can choose, and thus four bits are required to representthem. The values are 200, 210, 230, 250, 270, 290, 300, 320, 340, 360,380 and 390 milliseconds.

The fifth parameter which can be programmed is the coupled interval,represented by the letter T in FIGS. 13 and 14. The programmed valueserves no function if the pacer is programmed not to generate a secondstimulus at all. But if the pacer is programmed to generate it, theprogrammed coupled interval represents either a fixed time (if theprogramming disables scanning of the coupled interval), or it representsthe maximum coupled interval (if the programming calls for scanning ofthe coupled interval). In the latter case, the programmed coupledinterval is the first value used when the pacer functions for the firsttime after the initial programming; thereafter, the scanning begins withthe retained successful value. There are 15 coupled interval values fromwhich the physician can choose, 125, 140, 160, 180, 200, 210, 230, 250,270, 290, 300, 320, 340, 360 and 380 milliseconds, and thus four bitsare required to represent them.

The sixth parameter which can be programmed is "tachy rate"; this is theparameter which determines the width of the pulse generated bymonostable multivibrator MN1 (FIG. 9) each time that a heartbeat isdetected. Four bits are used to represent the tachy rate, and the eightpossible values are 40, 130, 140, 150, 165, 180, 200 and 225 beats perminute. For example, if a tachy rate of 150 beats per minute isselected, the pulse width of the multivibrator is adjusted such that theQ output of the multivibrator will remain high if, after any beat, foursuccessive beats are detected at a rate which exceeds 150 beats perminute, with the inter-beat interval between any two successive beatsnot exceeding 60/150 or 400 milliseconds.

The program controller chip IC5 responds to incoming reed switch, pulsesin four programming steps. Although the four steps will be described ina particular sequence, the first three can be interchanged; it is onlythe fourth step which must always be the fourth step in a programmingsequence.

There are seven output conductors from chip IC5, labeled A-F and L.These outputs are connected to seven inputs of chip IC 4, the inputsbearing similar letter designations. The seven inputs to chip IC4 appearat the top of FIG. 3, FIGS. 3 and 4 showing the details of chip IC4. TheA-D inputs are data bits which represent parameter values. The E and Finputs are address bits which select particular latches for the storageof the data bits. Input L is a latch control input. The programcontroller chip IC5 decodes incoming reed switch pulses, and applies twoaddress and four data bit values to its outputs A-F. The chip thenapplies a positive pulse to the L output which actually controlslatching of the data bits in a set of latches determined by the addressbits. (The program controller chip IC5 also sets an impedance value atits output pin 12, but this occurs during the fourth programming step,and will be described below.)

The first programming step involves setting the tachy rate. The four bitvalues which represent the rate and appear on conductors A-D are appliedto the D inputs of register flip-flops D1-D4 on FIG. 3. The E and Faddress bits are both low during this step, and consequently the outputof gate G1 is high to enable one input of gate G2. When the latch pulseis applied to the other input of gate G2 over conductor L, the gateoutput goes low. This output is coupled to the clock input of each ofthe four flip-flops. At the end of the latch pulse, the rising edge atthe clock input of each flip-flop clocks the four tachy-rate data bitsinto the four flip-flops.

The second programming step involves storing the four bits whichrepresent the initial delay in the latch which comprises registerflip-flops D7-D10 on FIG. 3. The four data bits on lines A-D areconnected to the D inputs of the flips-flops just as they are connectedto the D inputs of flip-flops D1-D4. It is now gates G7 and G8 whichcontrol latching of the data in the flip-flops. The output of gate G8 isconnected to the clock input of each of the flip-flops, and the latchinput pin 6 is connected directly to one input of gate G8, just as it isconnected to one input of gate G2. Just as the other input of gate G2 isconnected to the output of gate G1, the other input of gate G8 isconnected to the output of gate G7. One of the inputs of gate G7 isconnected directly to the F address input, and the other input of gateG7 is connected through an inverter to the E address input. Thus an EFaddress of 10 causes the output of gate G7 to go high, and at thetrailing edge of the latch pulse the initial delay data bits are clockedinto flip-flops D7-D10.

In the third programming step, the four data bits which represent thecoupled interval are latched into register flip-flops D11-D14. GatesG13, G14 control the latching of the data bits in these flip-flops justas gates G1, G2 and G7, G8 control latching of the data bits in the twoother sets of latches during the first and second programming steps. Itwill be noted, however, that both inputs to gate G13 are now derivedfrom inverters which are connected to the E and F address inputs.Consequently, in the third programming step the E and F outputs of chipIC5 are both high.

As mentioned above, the order in which the first three programming stepsoccur is unimportant, as long as the four data bits to be latched duringeach step are accompanied by E and F address bits which identify therespective one of the three sets of latches.

During the fourth (necessarily the last) programming step, all of therest of the programming information described generally above islatched. The sensitivity of the sense amplifier/comparator chip IC1 isdetermined by a single bit which controls an external resistorconnection to pin 12 of chip IC5. Pin 12 is coupled to the input filtercircuit for chip IC1, and it directly controls the sensitivity of chipIC1 as is standard in the pacer art. The reason that the fourthprogramming step must always be the last one is that a separate latch isnot provided for the sensitivity control. Chip IC5 itself serves as thesensitivity latch.

During the last programming step, the E and F address bits represent a01 combination. It will be noted that the E address bit input on FIG. 3is connected directly to one input of gate G3, and the F address bitinput is connected through an inverter to the other input of gate G3.The output of gate G3 is connected to one input of gate G4, the otherinput to which is connected to latch input pin 6. Consequently, gate G4clocks flip-flops D5 and D6 at the end of the latch pulse. The A and Bdata bits are connected to the D inputs of these two flip-flops, andthese two data bits represent the four pulse width values (off, and0.25), 0.35 and 0.6 milliseconds). The two pulse-width bits are storedin the two flip-flops during the last programming step.

The last two pieces of information required by the pacer are two bitswhich represent whether a second stimulus is to be generated at all and,if it is, whether the coupled interval is to be fixed or scanned. Thesetwo data bits appear at the C and D outputs of program controller chipIC5. Separate latches are not provided, and instead chip IC5 serves asthe latch for these bits just as it does for the sensitivity control.Referring to FIGS. 1 and 2, it will be noted that pins 9,10 of chip IC5are connected directly to pins 10,11 of chip IC3. Referring to FIG. 7(part of chip IC3), it will be seen that input pins 10,11 of chip IC3are connected to output pins 9,10 of chip IC5. If the program controllerchip IC5 maintains a low potential (a 0) at its output pins 9,10, thenonly a single stimulus is generated following each tachycardiaconfirmation. On the other hand, a high level (a 1) controls thegeneration of a second stimulus as well.

In a similar manner, output pin 11 of chip IC5 is connected to inputpins 16,17 of chip IC3 (see FIGS. 1 and 2). This connection is alsoshown on FIG. 7 (part of chip IC3). If the latched potential at outpinpin 11 of chip IC5 represents a 0, then the coupled interval is scanned,and if it represents a 1, the coupled interval remains fixed at thevalue latched in flip-flops D11-D14 (FIG. 4).

In the description of monostable multivibrator MN1 (FIG. 9) above, itwas explained that pins 1 and 2 of chip IC3 (see FIGS. 2 and 5) areconnected to the junction of capacitor C9 and a resistor chain. Theresistor chain is shown generally by the symbol R on FIG. 9, butactually comprises resistors R17, R18B, R18A, R19, R20 and R21 (see FIG.2). The tachy-rate flip-flops D1-D4 on FIG. 3 have their Q and Q outputsconnected to respective inputs of transmission gates TG1-TG4. Each gate,when turned on, shorts a pair of pins to each other, the five pins 7-11being connected to the various resistors in the resistor chain justdescribed. Thus if all of the transmission gates are off, all of theresistors are in the chain. On the other hand, when any two adjacentoutput pins are shorted to each other through a respective transmissiongate, the resistor or resistors connected between the two pins areshorted and do not contribute to the total impedance. It is in thismanner that the four tachy-rate flip-flops determine the minimum ratewhich must be exceeded for tachycardia confirmation, the physician beingable to select from among eight different rates (one of which is"artificial" in that it is not really a legitimate tachy rate, butrather is programmed in order to attempt to induce tachycardia).

Chip IC2 on FIG. 1 generates the stimulating pulses, as will bedescribed below. The width of each pulse is controlled by the potentialwhich appears at input pins 11,12. Pins 11,12 are connected to thejunction of resistors R14, R15 and R16. While resistor R14 is connectedto the positive supply rail, the other two resistors are connected tooutput pins 12 and 13 of chip IC4. These two output pins, together withoutput pin 23 and pulse-width flip-flops D5 and D6 (FIG. 3), determinethe pulse width.

Each of pins 12,13 and 23 is either floating or held at the potential ofthe positive supply (V_(DD) at pin 14, FIG. 3). If both of flip-flops D5and D6 have bits of value 1 stored in them, their Q outputs are bothhigh. Since both Q outputs are connected to inputs of gate G6, theoutput of gate G6 is low and the connected P-channel transistor betweenpins 14 and 23 is held on. Consequently, the positive potential at pin14 is extended to on/off pin 23. Referring to FIGS 1 and 2, it will benoted that pin 23 of chip IC4 is connected through resistor R7 to pin 2of chip IC2. Whenever the potential at pin 2 is high, no pulses aregenerated by chip IC2. Consequently, when a data bit combination of 11is stored in flip-flops D5 and d6, the device is inhibited fromoperating. For each of the other three combinations of data bits, theoutput of gate G6 is high and pin 23 floats. Pacer pulses can begenerated, and the pulse width depends on the potentials which appear atpins 12 and 13.

When each of flip-flops D5,D6 contains a 0, the two inputs of gate G5are low, and its output is high; the P-channel transistor connectedbetween pins 12 and 14 is off so pin 12 floats. Since the Q output offlip-flop D6 is high in such a case, the P-channel transistor betweenpins 13 and 14 is also off, and pin 13 floats. Referring to FIG. 2,resistors R15,R16 are effectively out of the circuit, and the onlyconnection to pins 11,12 of chip IC2 is that of resistor R14 whose otherend is connected to the positive supply rail.

With a 1 in flip-flop D5 and a 0 in flip-flop D6, pin 13 still floats.But the output of gate G5 is now low so that pin 12 is connected to thepositive supply at pin 14. Referring to FIG. 2, resistor R15 is noweffectively in parallel with resistor R14 between pins 11,12 of chip IC2and the positive supply.

The last case is that in which flip-flop D5 contains a 0 and flip-flopD6 contains a 1. The output of gate G5 is once again low due to one ofits inputs being connected to the Q output of flip-flop D6. The Q outputof the same flip-flop is low. Consequently, both of pins 12,13 areconnected to pin 14 through their respective coupling transistors.Effectively, all of resistors R14, R15 and R16 are connected in parallelbetween pins 11,12 of chip IC2 and the positive supply, to provide thethird possible pulse width.

Referring to FIG. 2, output pins 12,13,14,18 and 7 of chip IC3 will bedescribed below as selectively controlling the shorting out of resistorsin a series chain comprising resistors R9-R13. One end of the resistorchain is connected to ground (either through resistor R13, or directlythrough pin 12 of chip IC3 when the chip grounds the pin). The resistorchain then continues from the junction of pin 7 and resistor R9 toresistor R25. In a similar manner, resistors R22-R25 are connected inseries in the overall chain, with selected ones of the four resistorsbeing shorted out depending upon whether pin pairs such as 18,19 areinternally shorted in chip IC4. The resistor chain terminates inresistor R8 which is connected to capacitor C8. The resistor chain andthe capacitor control the timing of chip IC2, that is, when astimulating pulse is generated. The same resistor chain is used tocontrol the timing of both the first stimulus and the second stimulus(where required) and thus the same resistor chain determines both theinitial delay and the coupled interval. It is chip IC3 which shorts outselected resistors from among those in the group R9-R13 to controlscanning of both the initial delay and the coupled interval; asdifferent pairs of pins among pins 12,13,14,18 and 7 are shorted to eachother during the scanning of both the initial delay and the coupledinterval, both time periods decrease in 6-millisecond discrete steps.But the maximum time periods (when none of resistors R9-R13 are shorted)are controlled by chip IC4 and the selective shorting of resistorsR22-R25. The circuitry on FIG. 4 (part of chip IC4) selectively shortspairs of adjacent pins among pins 17-21 in order to control the maximuminitial delay and the maximum coupled interval. The control is exercisedby flip-flops D7-D10 or flip-flops D11-D14, depending upon whether it isthe initial delay or the coupled interval which is to be timed. The sameresistors are used for both types of control since the two types oftiming come into play at different times during each cycle.

Output pin 20 of chip IC3 (FIG. 2) is connected over the IPC conductorto input pin 15 of chip IC4. Output pin 19 of chip IC3 is connected overthe CPC conductor to input pin 16 of chip IC4. Both output pins arenormally high in potential. Whenever an initial delay is to be timed,chip IC3 causes IPC pin 20 to go low; conductor IPC is the initial-delaypulse control. Similarly, whenever a coupled interval must be timed,chip IC3 causes its pin 19 to go low; conductor CPC is thecoupled-interval pulse control. Referring to FIG. 4, when no timing isrequired, and both of the IPC and CPC conductors are high in potential,the outputs of all of gates G9-G12 and G15-G18 are low. Thus the inputsof all of gates G19-G22 are low, and all of these gate outputs are high.The high potentials hold off the four respective transmission gateswhich are connected between respective pairs of pins in the groupcomprising pins 17-21.

When an initial delay must be timed, conductor IPC goes low. Thus theIPC inputs to gates G9-G12 have no effect on the circuit operation, andthe gate outputs depend upon only the data stored in flip-flops D7-D10,since the Q outputs of these flip-flops are connected to respectiveinputs of the gates. Since the outputs of gates G15-G18 remain lowbecause the CPC conductor is high in potential, these gates do notaffect the operations of gates G19-G22. The latter gates are nowcontrolled by the outputs of gates G9-G12, that is, the data bits storedin flip-flops D7-D10. It is in this manner that this group of flip-flopscontrols the selective shorting of resistors R22-R25 to set the maximuminitial delay. During successive cycles, it is resistors R9-R13 whichare selectively shorted out so that the initial delay decreases in6-millisecond decrements. The combination of resistors R22-R25 which isinvolved in the initial delay timing is always the same whenever thetiming takes place, the combination being controlled by the data latchedin flip-flops D7-D10.

Similar remarks apply to the longest coupled interval (the only coupledinterval if there is no scanning). With the IPC conductor high and theCPC conductor low on FIG. 4, it is now flip-flops D11-D14 which controlthe operations of gates G19-G22 and consequently which of resistorsR22-R25 are placed in the resistor chain when coupled interval timing isrequired. The same pre-selected set of resistors is always used for thecoupled interval timing; the 6-millisecond decrements in the case of ascanned coupled interval are controlled by chip IC3 shorting out adifferent combination of resistors R9-R13 during successive cycles.

Brief Description Of Chips IC1 And IC2

Before proceeding to a detailed description of the overall system, itwill be helpful to review the operations of chips IC1 and IC2. Both ofthese chips (1438B and 1400R) are commercially available devices andthey perform standard functions. For this reason, it will suffice todescribe the input and output signals of the two chips, withoutdescribing how they work internally.

The two electrode connections (IND and STIM) are shown on the left sideof FIG. 1. The indifferent electrode is grounded. the stimulatingelectrode is coupled both to pins 20,21 of chip IC1 and to pins 9,10 ofchip IC2. Chip IC1 is a standard sense amplifier/comparator which servesto detect a heartbeat. As described above, the sensitivity is determinedby program controller chip IC5 (pin 12). The components connected tochip IC1 are all standard, and the sense amplifier/comparator operationis the same as that to be found in prior art pacers. Whenever aheartbeat is detected, a positive pulse appears at output pins 9,10.

Chip IC2 is a timing oscillator. It is the "heart" of a conventionalpacer, but is used in the illustrative embodiment of the invention onlyas a timer and pulse generator. A positive pulse appearing at pins 21,22is internally coupled through the chip to pins 19,20. The pulse iscoupled through capacitor C6 to pins 17,18. A trigger input at pins17,18 resets the internal oscillator in chip IC2 and starts a new timingcycle. Chip IC2 can operate in either the synchronous or the inhibitmode. In the former a stimulating pulse is generated at pins 9,10whenever a heartbeat is detected in order to reinforce it, and in thelatter such a reinforcement pulse is not generated. Because pin 1 isgrounded, chip IC2 operates in the inhibit mode.

If a positive potential is applied through resistor R6 to capacitor C6,the trigger pulses are not extended from pins 19,20 through thecapacitor. Thus when pin 6 of chip IC3 (FIG. 2) is high, it inhibits thedetection of heartbeats. A low potential applied to pins 17,18 alsoprevents the trigger inputs from resetting the timer. When reed switchRS1 is operated, the low potential applied through "hot carrier" diodeD2 to pins 17,18 causes the oscillator in chip IC2 to run free andpacing pulses to be generated continuously. (The term "hot carrier"refers to the fact that the voltage drop across the diode is 0.3 volts,not the usual 0.6 volts.) The pulses are in fact not generatedcontinuously, but the reason will be described below.

Pacing pulses are generated at pins 9,10 of chip IC2, and are coupledthrough capacitor C5 to the stimulating electrode. Coincident with eachpacing pulse, a negative pulse is generated at pins 3,4.

A negative pulse is also generated at pins 15,16 whenever a pacing pulseis delivered to the stimulating electrode, just as a negative pulseappears at pins 3,4. However, a negative pulse also appears at pins15,16 whenever a heartbeat is detected, in which case a negative pulsedoes not appear at pins 3,4 since chip IC2 is operated in the inhibitmode. Capacitor C4 is the charge storage capacitor which dischargesthrough pins 9,10 whenever a stimulating pulse is required. CapacitorC8, connected between pin pairs 15,16 and 13,14 is the rate timingcapacitor. This capacitor, as well as resistor R8 and all of theresistors previously described in the resistor chain, determine the rateat which the internal oscillator of chip IC2 operates.

The potential at pins 11,12 of chip IC2 controls the width of each pulsewhich is generated, as described above.

Lastly, a high potential applied to pin 2 of chip IC2 disables the chipfrom generating pacing pulses at all. When pin 23 of chip IC4 (FIG. 2)is high in potential, as described above, the potential extended overthe on/off conductor and through resistor R7 prevents pacing pulses frombeing generated. Capacitor C7 is normally charged through resistors R26,R4 so that it also normally inhibits pulse generation. Chip IC2 is thusheld off most of the time. When a stimulus is required, capacitor C7 isdischarged through diode D3 and resistor R31, as will be describedbelow.

With these remarks in mind, the system operation will now be described.The system logic is controlled by chip IC3. In the following descriptionreference should be made to FIGS. 5-8 (chip IC3), as well as to FIGS. 1and 2 which depict the connections from chip IC3 to the remainder of thesystem. It should be noted that two pin connections to chip IC3 are notshown. These pin connections are merely test points and are not involvedin the system operation; they are omitted from the drawing for the sakeof clarity. Pin 3 of chip IC3 is in fact connected to the Q output ofmultivibrator MN1, shown in block form on FIG. 5 and in detail on FIG.9. Pin 9 of chip IC3 is the output of an inverter, also not shown, whoseinput is connected to the output of inverter 59A at the lower rightcorner of FIG. 8.

System Operation

When a heartbeat is detected, a negative pulse appears at pins 15,16 ofchip IC2, as described above. This pulse is extended to pin 5 of chipIC3, as shown in FIGS. 1 and 2. The negative pulse is inverted byinverter 1 (FIG. 5) and a positive pulse is applied to the trigger (A)input of the monostable multivibrator. A positive pulse now appears atthe Q output of the multivibrator, its duration being dependent upon the"tachy rate" programmed by the physician (see description above of chipIC4 and resisters R17-R21). The Q output is connected to one input ofgate 4. The same pulse which triggers the multivibrator is applied to asecond input of gate 4. The third gate input is connected to the outputof inverter 7B which is normally high in potential. Thus as long as theoutput of inverter 7B is high, the output of gate 4 is pulsed lowwhenever a heartbeat is detected.

Flip-flops 5,6 and 7 comprises a standard ripple counter which isinitially reset to 000. With the Q output of each of flip-flops 5 and 7initially high, and since they are connected to inputs of gate 7A, thegate output is low. The output is inverted by inverter 7B to apply ahigh potential to the third input of gate 4.

Flip-flop 5 is toggled on the trailing edge of each output pulse fromgate 4. If the counter is not reset, as successive heartbeats aedetected and the counter cycles from 000 to 100, the Q output of atleast one of fip-flops 5 and 7 remains high, and the output of gate 7Aremains low. But when the fifth pulse is counted without the counterhaving been reset during the sequence, the Q output of each offlip-flops 5 and 7 is low, and the output of gate 7A goes high. Theoutput of inverter 7B now goes low to disable gate 4; no further pulsesare counted.

The Q output of multivibrator MN1 is connected to an input of gate 10.Whenever the multivibrator times out, that is, the Q output goes lowwithout the output pulse being extended by the arrival of anothertrigger input before the time-out is over, one input to gate 10 goeslow. The output of gate 7A is connected to the other input of gate 10,and this input is thus low in potential until five heartbeats have beencounted. Thus each time-out of the multivibrator, as long as the counterhas not reached a count of five, causes the output of gate 10 to gohigh.

One input of gate 9 is connected to the output of inverter 14, whoseinput is connected to the output of gate 37. As will be described below,the output of gate 37 is normally high, and thus one input to gate 9 isnormally low. Consequently, whenever the Q output of the multivibratorgoes low at the end of a time-out and the output of gate 10 goes high,the output of gate 9 goes low, and the output of inverter 9A goes high.Since the gate output is connected to the reset input of each flip-flopin the counter, this causes the three-stage counter to reset to 000.

Thus whenever a heartbeat occurs after a preceding heartbeat with aninter-beat interval longer than the reciprocal of the "tachy rate", thecounter is reset and the tachycardia confirmation cycle starts all overagain. But if five rapid heartbeats are detected in succession, the Qoutput of the multi-vibrator does not go low to reset the counter. Eventhough it may go low after the fifth beat is counted, the output of gate7A is now high and it is connected to an input of gate 10; thus theoutput of inverter 9A is locked low as soon as a count of five isreached so that the counter cannot be reset even if the multi-vibratortimes out.

The tachycardia confirmation test involves four rapid beats, not five,even though five beats are counted. The first beat merely serves as atime reference for the second. The basic test is whether four rapidbeats occur in succession, each of which is too soon after therespective previous beat. Once tachycardia is confirmed, the counterremains at a count of five and further counting is inhibited. The lowpotential which is now at the output of inverter 7B holds gate 4 off.

This same potential is inverted by inverter 3 and thus a positivepotential appears at pin 6 of chip IC3 (FIG. 5). As indicated on theleft of FIG. 5, and as shown in FIGS. 1 and 2, the positive potential isextended through resistor R6 to pins 19,20 of chip IC2. Any furtherheartbeats which are detected by chip IC1 are thus ignored. Also, sincethe count of five was reached in the first place by a negative pulseappearing at pin 5 of chip IC3, which pulse resulted from chip IC2having detected a heartbeat and generated a negative pulse at pins15,16, the oscillator on chip IC2 starts timing a new cycle. As willbecome apparent, this timing determines the initial delay, followingwhich chip IC2 generates a first stimulus. The reason for inhibitingheartbeat detection in chip IC2, by holding pins 19,20 high as justdescribed, is that the oscillator on chip IC2 is used to determine whenthe stimuli should be applied, and this timing function should not beinterfered with by any heartbeats which may occur.

When the output of gate 7A first goes high, several things happen inaddition to those described above. First, gate 29 is enabled since oneof its inputs is now high. (Its other input, however, is still low.)Second, the positive potential is inverted by inverter 7C, and invertedonce again by inverter 58B to clock flip-flop 58. since the D input ofthe flip-flop is connected to the positive supply, the flip-flop is setand its Q output goes high to enable gate 57. Third, the positivepotential which now appears at the output of inverter 59A is applied tothe second input of gate 57 and also to the gate of transistor 56. Thetransistor turns on, and the output of gate 57 goes low.

The output of gate 57 is the IPC conductor which, as shown on FIG. 2, isextended from pin 20 of chip IC3 to pin 15 of chip IC4. It will berecalled that when the IPC conductor goes low, chip IC4 (FIGS. 4 and 5)shorts out pre-selected ones of resistors R22-R25 for controlling theprogrammed (longest) initial delay. It will also be recalled thatcapacitor C7 on FIG. 1 is initially charged to a positive potential, thepositive potential at pin 2 of chip IC2 preventing the generation ofstimulating pulses. Now that a stimulus is required, however, a lowpotential must be applied to pin 2 of chip IC2. Because conductor IPC isnow low in potential, capacitor C7 discharges through diode D3 andresistor R31 so that a stimulus can be generated.

Referring to FIGS. 1 and 2, the overall resistor chain involved in alltiming functions of chip IC2 consists of resistors R9-R13, R22-R25 andR8, different ones of the resistors having shorted out at differenttimes. With transistor 56 on FIG. 8 now on, pin 12 of chip IC3 isgrounded. As shown on FIG. 2, this shorts our resistor R13 from theresistor chain. The actual initial delay which is now timed depends upontwo sets of resistors, R9-R12 and R22-R25. The latter set ispre-selected and the same resistors are always placed in the chainwhenever an initial delay is to be timed. If all of the resistors R9-R12are included in the chain, then the pre-selected combination ofresistors R22-R25 provides the longest initial delay, as programmed bythe physician. But the actual initial delay in any cycle is determinedby which of resistors R9-R12 happen to be shorted, i.e., how many6-millisecond decrements have already taken place. Depending upon thetotal impedance of the resistor chain, the oscillator on chip IC2 timesout and results in the generation of a first stimulating pulse.Coincident with this pulse, and as described above, a negative pulse isgenerated at pins 3,4 on chip IC2. This pulse is coupled throughresistor R30 on FIG. 1 to pin 8 of chip IC3. As shown on FIG. 8, thenegative pulse at pin 8 is inverted by inverter 68 and thus resetsflip-flop 58. Gate 57 now turns off, and it is gate 59 whose CPC outputnow goes low.

As described above, when the CPC conductor goes low, a differentcombination of resistors R22-R25 is included in the resistor chain.Since the oscillator on chip IC2 is still free running as a result ofpins 19,20 being held high (assuming that the second stimulus is to begenerated), the coupled interval timing now takes place. Chip IC3selects some other combination among resistors R9-R12 depending on howmany decrements of the coupled interval have already taken place, aswill be described below, but the resistors controlled by chip IC4 andthe CPC signal (FIG. 4) are such that should all of resistors R9-R12 beincluded in the chain, the longest coupled interval will be timed. Atthe end of the interval, a second stimulus is generated.

Chip IC2 can generate a second pulse only if pin 2 is not held high todisable pulse generation. It is the IPC conductor going low whichdischarges capacitor C7 rapidly to permit the first pulse to begenerated. Although the IPC conductor goes high when the CPC conductorgoes low, capacitor C7 charges through the high-impedance resistor R4.The capacitor cannot charge fast enough to inhibit the generation of asecond pulse even for a coupled interval of maximum duration.

Assuming that a second pulse is to be generated, pins 10,11 on FIG. 7are high in potential as described above. As soon as the Q output offlip-flop 58 goes high upon tachycardia confirmation, both inputs ofgate 47 are high, its output goes low, and the output of inverter 47Agoes high to reset flip-flop 45. The low potential at the Q output ofthe flip-flop disables gate 37, whose output remains high. The flip-flopoutput is initially high because the output of inverter 46 is low, theinput to the inverter normally being held high by the high potential atpins 3,4 of chip IC2. Even though the negative input pulse at pin 8,which pulse is coincident with the first stimulus, is inverted byinverter 46 so that a positive pulse is applied to the other input ofgate 37, the gate output remains high since flip-flop 45 is still reset.

The negative pulse at pin 8 is coupled to one input of gate 49. Sincethe other input to the gate is connected to the low Q output offlip-flop 45, the output of gate 49 goes high with the generation of thefirst stimulus. A negative pulse thus appears at the output of inverter48, and flip-flop 45 is clocked on the trailing edge of the pulse; bythis time flip-flop 58 has been reset so as to lift the reset fromflip-flop 45. The Q output of flip-flop 45 thus goes high at the end ofthe pulse at pin 8, after the short switching time of the flip-flop.Although one input of gate 37 is thus now held high, the output ofinverter 46 is low once again since the pulse at pin 8 has terminated.Thus the first stimulus results in the setting of flip-flop 45 but theoutput of gate 37 remains high.

The pulse at pin 8 which is coincident with the second stimulus has noeffect on flip-flop 45, the flip-flop remaining set until the nexttachycardia confirmation at which time flip-flop 58 is set once againand gate 47 causes flip-flop 45 to reset. But the second pulse at pin 8,through inverter 46, causes the output of gate 37 to now go low and theoutput of inverter 14 to go high. The output of gate 9 thus goes low andthe output of inverter 9A goes high in order to reset the ripple counterwhich comprises flip-flops 5-7. Since two pulses have been delivered,the system now starts looking for a tachycardia episode all over again,in order to determine whether another pair of pulses must be generated.Toward this end, monostable multivibrator MN1 is reset by the negativepulse at the output of gate 37, after inversion by inverter 2A.

If only a single stimulus is to be generated, pins 10,11 on FIG. 7 arelow. Consequently, the output of gate 47 is high and the output ofinverter 47A is low so that flip-flop 45 is not reset by the setting offlip-flop 58. The Q output of flip-flop 45 remains permanently high. Thefirst negative pulse at pin 8, which pulse is coincident with the firststimulus, results in the output of gate 37 going low. The 3-bit countercomprising flip-flops 5-7 is thus reset after the first stimulus isgenerated. Flip-flop 58 is also reset by the first negative pulse at pin8 and its Q output goes high to enable one input of gate 59. However,the other input is derived from the output of gate 7A which now goes lowonce again with the resetting of flip-flops 5 and 7. Consequently, eventhough flip-flop 58 is reset, the output of gate 59 does not go low; theCPC conductor remains high and there is no timing of a coupled interval.

Reed switch RS1 on FIG. 1 is connected to pin 15 of chip IC3. Referringto FIG. 6, it will be noted that each time the reed switch is operatedand a ground potential appears at pin 15, inverters 26 and 26A applypositive reset pulses to all of flip-flops 22-25 and 60-63. As will bedescribed below, these are the flip-flops which control the decrementingof the initial delay and the coupled interval by 6-milliseconddecrements. During programming, each time the reed switch is operatedall of the flip-flops are reset. This has the effect of inserting all ofresistors R9-R12 (FIG. 2) in the resistor chain so that the longest(programmed) initial delay and coupled interval are first timed.Whenever a cycle does not result in tachycardia termination, flip-flops22-25, which are arranged as a four-bit counter register, have theircount incremented so that in the next cycle the initial delay isdecremented by 6 milliseconds. After the fifteenth decrement, theinitial delay is set to its highest value once again, as the countercycles from 1111 to 0000. On alternate resettings of flip-flops 22-25,the similar counter register which comprises flip-flops 60-63 isincremented so that the coupled interval is decremented by 6milliseconds.

It is gate 21 which controls the incrementing of the counter whichcomprises flip-flops 22-25. The count representing the number of6-millisecond decrements of the initial delay is incremented wheneverthe output of gate 21 goes high. It is important that gate 21 notoperate immediately after the one or two required stimuli are generated.That is because if tachycardia has been terminated, the count inflip-flops 22-25 should be retained so that the same initial delay andcoupled interval values will be used when the next tachycardia episodeis confirmed; downward scanning of the initial delay and the coupledinterval always begin with the two last successful values. (It is onlywhen a tachycardia episode is encountered following initial programmingthat the scanning begins with the maximum initial delay and the maximumcoupled interval, since all of flip-flops 22-25 and 60-63 are reset.)

Gates 15-20 comprise a standard D-type flip-flop. The output of gate 17is the Q output of the flip-flop, and the output of gate 20 is the Qoutput. The set input, applied to inputs of gates 16 and 19, is derivedfrom the Q output of multivibrator MN1, and the reset input is derivedfrom the output of gate 37. The reason for the rather complicated formof flip-flop is that it must be set by the rising edge of the pulse atthe Q output of the multivibrator, and the rising edge is notnecessarily sharp; the flip-flop which is used, standard in the art, canbe set even on a slowly rising edge.

The flip-flop is reset when the output of gate 37 goes low. This isafter the first stimulus has been delivered if the pacer has beenprogrammed not to deliver a second, or after the second stimulus hasbeen delivered if the pacer has been programmed to deliver a secondstimulus as well as a first. When the flip-flop resets, the Q output(output of gate 17) goes low, this output serving as one input to gate21. The output of inverter 7B is connected to a second input of gate 21.This output is low during the initial delay and coupled interval timingperiods, but when gate 37 controls the resetting of the flip-flopcomprising gates 15-20, it also controls resetting of the countercomprising flip-flops 5-7. As soon as the latter flip-flops reset, theoutput of gate 7B goes high. Thus, the output of gate 21 remains loweven though the output of gate 17 no longer holds it low.

Gate 21 should not operate to increment the counter which comprisesflip-flops 22-25 because when the tachycardia confirmation circuit isfirst enabled to operate once again, there is no way of knowing whethertachycardia has yet been terminated. If it has been terminated, theoutput of gate 21 should remain low so that flip-flop 22 is not toggled.In the event the output of gate 17 goes low before the output ofinverter 7B goes high, two inputs to gate 21 would be low, and theoutput would go high to toggle flip-flop 22. In order to prevent this,the output of gate 37 is coupled through inverter 14 to a third input ofgate 21. While the output of gate 37 is low the output of inverter 14 ishigh, so that the output of gate 21 remains low. By the time the outputof gate 37 goes high once again, the output of inverter 7B has gone highso that it can hold the output of gate 21 low.

Thus by the time that the output of gate 37 reverts to its normally highstate, the tachycardia confirmation circuit is enabled to operate onceagain, and the flip-flop which comprises gates 15-20 is reset with theoutput of gate 17 being low. If tachycardia has not been terminated, themultivibrator MN1 does not time out as it is continuously re-triggeredby heartbeats which are once again detected (since pin 6 on FIG. 5 isnow low), and the Q output remains low after the first multivibratortriggering. Consequently, following the next tachycardia confirmation,when the output of inverter 7B goes low, all three inputs to gate 21 arelow in potential and the output goes high to clock flip-flop 22. Sincetachycardia has not been terminated, the initial delay which is nowtimed is decremented 6 milliseconds.

On the other hand, if tachycardia has been terminated, the multivibratortimes out and the Q output goes high. The flip-flop comprising gates15-20 is now set and the output of gate 17 goes high. Thus the output ofgate 21 is held low. Even though another tachycardia episode may beconfirmed some time later, when the output of inverter 7B goes low itdoes not result in the toggling of flip-flop 22. This allows thepreviously successful initial delay and coupled interval to be the firstones which are used.

It will be recalled that immediately upon tachycardia confirmation, theoutput of gate 7A goes high to enable one input of gate 29 (FIG. 5). Theother input to this gate is connected to the output of gate 59, the CPCconductor, which is high in potential during the initial delay timing.Thus the output of gate 29 is low, and it enables the operation of eachof gates 30-33. The outputs of these four gates are controlled byrespective flip-flops 22-25, and the output of each of gates 30-33 iscoupled to an input of a respective one of gates 39-42. Each of theselatter gates has another input, but these other inputs have no effectduring the initial delay timing. The CPC conductor which is high inpotential causes the output of each of gates 50-53 to remain low.

The outputs of gates 39-42 are coupled to respective transmission gates28,35,44 and 55. As seen on FIG. 2, these are the four gates whichcontrol the selective shorting of resistors R9-R12 at pins 7, 18, 14 and13 of chip IC3. (Gates 28 and 35 each includes a P-channel and anN-channel transistor connected in parallel; because these gates controlresistors in the middle of the resistor chain, a full drive may not beavailable to fully turn on a single N-channel device. By providing twoopposite-type transistors in parallel, they compensate for each other,as is known in the art. Single-transistor gates 44 and 55 are sufficientto short out resistors R11 and R12 since these resistors are at the endof the chain, closer to ground potential.)

When flip-flops 22-25 represent a count of 0000, all of resistors R9-R12are in the resistor chain. The resistors are weighted in the approximateratio 1:2:4:8 so that as flip-flops 22-25 count in binary fashion,successive decrements of the initial delay are all the same.

Referring to FIG. 2, it will be recalled that resistor R13 is shortedout by transistor 56 (FIG. 8) immediately upon tachycardia confirmation.Resistor R13 is nominally 10M. In the absence of tachycardia, thisartificially high resistor is placed in the resistor chain in order tomake the time-out period of the oscillator in chip IC2 so high that nopacing pulses can be generated; even though pin 2 of chip IC2 is heldhigh in the absence of tachycardia to prevent the generation of pacingpulses, chip IC2 also requires a resistive connection to pins 13,14. Butwhen one or two stimuli must be generated, resistor R13 is removed fromthe circuit so that the only resistors which control initial delay andcoupled interval timing are resistors R9-R12, R22-R25 and R8. The reasonfor providing resistor R8 is that if a minimum initial delay or coupledinterval has been programmed, all of resistors R22-R25 are shorted out,and if all of resistors R9-R12 are similarly shorted out at the end ofthe scan of the initial delay or coupled interval, then there would beno resistance connected to pins 13,14 of chip IC2. Resistor R8 serves asthe minimum resistance for controlling a minimum initial delay orminimum coupled interval when the counter which comprises flip-flops22-25 or the counter which comprises flip-flops 60-63 counts all the wayup to 1111 and shorts out all of resistors R9-R12.

If a second stimulus is to be provided, then as described above the CPCconductor (output of gate 59 on FIG. 8) goes low after the firststimulus is generated. The output of gate 29 is now high, the outputs ofall of gates 30-33 are low, and thus flip-flops 22-25 have no effect onthe outputs of gates 39-42. But because the CPC input to each of gates50-53 is now low, the outputs of these gates are determined by the countcontained in flip-flops 60-63. It is now these four flip-flops whichdetermine which of resistors R9-R12 are included in the resistor chainfor controlling the coupled interval.

Flip-flops 60-63 control the scanning of the coupled interval. Thepotential at pins 10,11 (FIG. 7) has already been described ascontrolling whether or not a second stimulus takes place at all. Thedescription thus far has also taken into account the timing of thecoupled interval in accordance with the count in flip-flops 60-63. Thereremains to consider how these flip-flops are cycled.

Cycling is not required at all if a fixed coupled interval is to beemployed. In such a case, pins 16,17 (FIG. 7) are high in potential andthe output of gate 67 is low. The output of gate 67A remains high tohold flip-flop 66 reset. Since the Q output of flip-flop is high, theoutput of gate 65 remains low. The output of gate 65 never exhibits afalling edge and flip-flop 60 is never toggled. All of flip-flops 60-63are reset when the pacer is programmed. Consequently, all of resistorsR9-R12 remain in the resistor chain during the coupled interval timing,and the coupled interval remains fixed at the programmed value. On theother hand, if the coupled interval is to be scanned, pins 16,17 are lowin potential so that flip-flop 66 is not held reset and the output ofgate 65 is not held low. The flip-flop is initially reset, however,following programming; the low potential at pin 15 (FIG. 6) when thereed switch closes is inverted by inverter 26A to control resetting offlip-flop 66 along with resetting of flip-flops 60-63.

In the presence of normal heartbeats, the output of gate 7A is low.Similarly, multivibrator MN1 keeps on timing out since heartbeats areoccurring at a rate slower than the tachy rate; when the Q output of themultivibrator goes high at the end of each time-out, a set pulse isapplied to the flip-flop comprising gates 15-20. The flip-flop is notreset because the output of gate 37 remains high, and thus the output ofgate 20 remains low. Thus the output of gate 36 is high to enable aninput of gate 67 so that flip-flop 66 remains reset.

Upon tachycardia confirmation, the output of gate 7A goes high and thusthe output of gate 36 goes low so that the reset input of flip-flop 66is no longer forced high. Assuming that tachycardia is not terminated,succcessive single pulses or successive double pulses are generated insuccessive cycles, and the output of gate 37 goes low at the end of eachcycle. The flip-flop comprising gates 15-20 is continuously reset and,because the flip-flop is not set by the Q output of multivibrator MN1going high, each time that the output of gate 7A goes high upontachycardia confirmation gate 21 increments the count in flip-flops22-25. The initial delay is scanned down to its minimum value, at whichtime flip-flops 22-25 represent a count of 1111. The four inputs of gate38 are connected to the respective Q outputs of the four flip-flops, andat this time the output of the gate goes low. Although the output ofgate 38 is coupled to one input of gate 65, the other input of gate 65is connected to the Q output of flip-flop 66 which is high since theflip-flop is still reset. Consequently, the output of gate 65 stillremains low.

If tachycardia is still not terminated, when the output of gate 7A nextgoes high gate 21 advances the count in flip-flops 22-25 from 1111 to0000, and the output of gate 38 goes high once again. The positive stepat the output of gate 38 clocks flip-flop 66 since it is applieddirectly to theC input and through inverter 66A to the C input. Theflip-flop is now set and the Q output goes low. But the output of gate65 still remains low since the output of gate 38 is now high.Consequently, another scan of the initial delay begins with theprogrammed value, without the count represented in flip-flops 60-63being incremented.

During the last cycle of the next scan of the initial delay, however,when flip-flops 22-25 represent a count of 1111 and the output of gate38 is low, both inputs to gate 65 are low and its output is high. Iftachycardia is not terminated during this cycle, gate 7A goes high inthe usual way upon the next tachycardia confirmation. As soon asflip-flops 22-25 are cycled from 1111 to 0000 to begin a new scan of theinitial delay, the output of gate 38 goes high once again and now theoutput of gate 65 goes low to exhibit a falling edge. This results inthe clocking of flip-flop 60 and decrementing of the coupled interval by6 milliseconds. When the output of gate 38 thus goes high for the secondtime, flip-flop 66 is clocked once again and it is now reset with the Qoutput going high. This holds the output of gate 65 low at the start ofthe next scan of the initial delay so that the coupled interval is notdecremented even though gate 38 pulses once again. The net result isthat the coupled interval is decremented by 6 milliseconds only at thestart of every other scan of the initial delay.

The reason for this is that when the patient's heart has been beatingnormally but tachycardia is then confirmed, the scanning begins with theretained values of the initial delay and the coupled interval, stored inrespective flip-flops 22-25 and 60-63. If tachycardia is not terminated,the initial delay is scanned down to the minimum value while the coupledinterval remains at the previously successful value. Were the coupledinterval to be decremented at the end of the first partial scan of theinitial delay, there would be no scan of the higher value initial delayswith the previously successful coupled interval. The first time that themaximum initial delay would be utilized at the start of the firstcomplete scan, the coupled interval would be decremented and thepreviously successful value of the coupled interval would not be used atall until both the initial delay and the coupled interval would bescanned back to the point at which the coupled interval would be at thepreviously successful value. It is for this reason that the coupledinterval is not decremented at the end of the scanning of the initialdelay from the previously successful value to the minimum value. Afterthis partial scan, a complete scan of the initial delay is controlledwhile using the previously successful coupled interval. It is only afterthis complete scan of the initial delay that the coupled interval isdecremented.

The same operation ensures whether tachycardia is terminated during ascan of the initial delay which began with decrementing of the coupledinterval, or during a scan of the initial delay at the beginning ofwhich the coupled interval was not decremented. It makes no differencebecause upon tachycardia termination the output of gate 20 goes lowwhile the output of gate 7A is low, and the output of gate 36 goes highto reset flip-flop 66.

It should be noted that the mechanism by which flip-flop 66 controlsdecrementing of the coupled interval only after every other completescan of the initial delay is not really necessary during most of thecycling. It is only at the beginning of an overall scanning sequencethat the coupled interval should not be decremented when flip-flops22-25 are clocked to represent a count of 0000 for the first time.Thereafter, it is not necessary to control decrementing of the coupledinterval only at the start of every other scan of the initial delay. Itwould be feasible, if desired, to allow the coupled interval to bedecremented at the start of every scan of the initial delay, after thereis at least one full scan of the initial delay with the previouslysuccessful coupled interval.

Most conventional heart pacers are designed so that a physician candetermine the battery potential in order that the remaining life of thepacer may be ascertained. Often this is accomplished by placing a magnetover the patient's chest in the vicinity of the pacer, whereupon theclosing of a reed switch causes the pacer to generate pulses at acontinuous rate which is dependent upon the battery potential. Butcontinuous pulses are not generated by a tachycardia control pacer. Thusthere is no apparent way for the physician to determine the batterypotential.

It would also be advantageous were there some way for the physician toascertain the programmed values of initial delay and coupled interval.This is especially true in the case of a patient who consults aphysician other than the one who programmed the device, in which casethere may be no record of the programmed values. While the physiciancould monitor the ECG waveform of the patient and measure the maximum(programmed) values of initial delay and coupled interval, there is aproblem with this approach. Even assuming that tachycardia can beinduced so that stimuli are generated, a complete scanning cycle, thatis, a complete scan of the initial delay and the coupled interval,typically takes longer than 10 minutes (allowing for tachycardiaconfirmation following each cycle). Since the scanning does notnecessarily begin with the maximum initial delay and coupled interval,the physician may actually have to observe an ECG waveform for more than10 minutes before the maximum initial delay can be ascertained. It wouldbe highly desirable to provide a mechanism by which the programmedvalues could be determined rapidly.

There is one other capability which would also be advantageous and thatis to provide a simple mechanism whereby the patient can completelyinhibit operation of the pacer. The physician can accomplish this byprogramming the device so that pin 23 of chip IC4 (on/off) is high inpotential, as described above. But if the patient is feeling discomfort,it is also advisable to provide him with a simple mechanism fordisabling the pacer operation until the physician can program it off.

Flip-flops 12,13, transistor 8, and the several gate connections to pin15 on FIG. 6 allow all of the aforesaid desirable features to be addedto the pacer at little additional cost and with a minimum of complexity.

The reset inputs of flip-flops 12,13 on FIG. 6 are connected to pin 15.Thus during normal operation when reed switch RS1 (FIG. 1) is open, apositive potential is applied to the reset inputs of the flip-flops andthey are held reset. But if a magnet is applied to close the reedswitch, pin 15 is grounded through the switch so that the reset input tothe flip-flops is lifted. The closing of the reed switch also serves twoother functions. The first is to allow capacitor C7 to discharge throughresistor R4. It will be recalled that the capacitor is normally chargedthrough resistors R4 and R26 to prevent any pacing pulses from beingdelivered, the capacitor being discharged through diode D3 and resistorR31 when the IPC conductor goes low following tachycardia confirmation.In the same way, capacitor C7 discharges through resistor R4 and thereed switch to allow chip IC2 to generate pulses. Although resistor R4is large in magnitude and capacitor C7 does not discharge quickly whenthe reed switch is closed, that is of no moment; as will be described,the desired operation is the generation of a pair of pulses and what isimportant is the time between the two pulses, not when the first oneoccurs. (It should be observed that if the on/off conductor is high inpotential, capacitor C7 remains charged through resistor R7 which is ofmuch lower magnitude than resistor R4, and chip IC2 cannot generate anypulses even if the reed switch is closed. If the pacer has beenprogrammed off, it remains off even if the reed switch is closed by amagnet.)

The other function preformed by the reed switch is the pulling low ofpins 17,18 of chip IC2 through diode D2. When these pins go low, chipIC2 operates in a free-running mode.

Eventually, capacitor C7 discharges through resistor R4 and chip IC2starts to deliver pacing pulses at pins 9,10. With the delivery of eachpulse, pins 3,4 go low as described above. In the usual way, a negativepulse is applied to pin 8 of chip IC3 (FIG. 8). Each negative pulse isinverted by inverter 11B and thus a positive pulse is applied to oneinput of gate 11 on FIG. 6. With flip-flops 12,13 being initially reset(as a result of pin 15 having previously been held high when the reedswitch was open), the Q output of flip-flop 13 is high and thus thesecond input of gate 11 is enabled. When the output of gate 11 is pulsedlow with the delivery of the first pulse from chip IC2, flip-flop 12 isset on the trailing edge. Flip-flops 12,13 comprise a standard two-bitripple counter. The next pacing pulse results in the clocking offlip-flop 12 once again, since the Q output of flip-flop 13 is stillhigh to enable gate 11 when the pulse arrives. But at the trailing edgeof the pulse, when flip-flop 12 is reset and flip-flop 13 is set, the Qoutput of the latter flip-flop goes low to disable gate 11. At the sametime, gate 8 turns on and applies the positive battery potential to pin4. Referring to FIGS. 1 and 2, it will be seen that this positivepotential charges capacitor C7 through resistor R7, just as does pin 23of chip IC4 when the pacer is programmed off. Consequently, chip IC2delivers only two pulses at pins 9,10.

The time interval between the two pulses is controlled in the usualmanner by the resistor chain connected to pins 13,14 of chip IC2. Noneof the resistors in the overall chain is shorted out. It will berecalled that pin 12 of chip IC3 (FIG. 2) shorts out resistor R13 duringinitial delay and coupled interval timing. The resistor is shorted outwhen pin 12 on FIG. 8 goes low under control of transistor 56, thetransistor being turned on when the output of gate 7A goes high upontachycardia confirmation. But there is no tachycardia confirmation now,so gate 56 remains off and resistor R13 is not shorted out. The lowpotential at pin 15 of chip IC3 (see FIGS. 1, 2 and 6) due to theclosing of the reed switch applies a positive potential throughinverters 26,26A on FIG. 6 to the reset input of each of flip-flops22-25 and 60-63. With all eight flip-flops reset, the inputs to all ofgates 39-42 are low, and all of the gate outputs are high to hold offgates 28, 35, 44 and 55. Consequently, all of resistors R9-R12 aresimilarly placed in the resistor chain.

Both of the IPC and CPC conductors on FIG. 8 are high since there hasbeen no tachycardia confirmation. Referring to FIG. 4, the inputs of allof gates G19-G22 are low, all of the gate outputs are high, and thusnone of pins 17-21 are shorted to each other. Consequently, all ofresistors R22-R25 remain in the resistor chain.

The net result is that the time interval between the two pulsesgenerated by chip IC2 is the maximum, and is determined primarily byresistor R13. This maximum is selected so that the battery potential of2.8 volts controls an inter-pulse interval of 1.5 seconds. As thebattery potential decreases with age, the inter-pulse interval isreduced proportionally since it takes longer for capacitor C8 (FIG. 1)to charge. All the physician has to do is to observe the patient's ECGwaveform and to time the interval between the two pulses in order toascertain the battery potential. This is similar to the prior arttechnique of using a magnet to control the rate of a conventional heartpacer in order to determine the battery potential, inasmuch as the timeinterval between the two pulses which are generated is equivalent to a"rate". Of course, in order to achieve the effect with a tachycardiacontrol pacer, it is necessary to artificially control the generation ofat least two pulses in the manner described, even through chip IC2 doesnot function as an ordinary pacer.

It should be noted that a relatively high value of resistance is usedfor resistor R13 in order that the inter-pulse interval will varybetween approximately 1.5 and 1.7 seconds as the battery ages. Twopulses which occur this far apart (the separation should be at least onesecond) can have no deleterious effect on the beating of the patient'sheart.

The physician can use a magnet in the manner described in order todetermine the battery potential. (In general, the time interval betweenthe two pulses could represent some preselected pacer characteristicother than battery potential, e.g., the number of tachycardia espisodeswhich have occurred if a suitable counter is provided.) But the patientcan also use such a magnet to shut off the pacer so that it does notgenerate pulses even following tachycardia confirmation. Since pin 2 ofchip IC2 is held at a high potential after two pulses are generated, foras long as the magnet is applied, the patient can hold the pacer off byholding the magnet in place. He may then go to see his physician (whilestill holding the magnet in place to keep the pacer off), and thephysician can program the pacer permanently off by forcing pin 23 ofchip IC4 high as described above. (Along the same lines, the patientmight be furnished with a programmer of his own which would only becapable of programming the pacer off. Only the physician's programmercould control programming of the pacer on once again. A patient-operatedpace-maker programmer of this type, although used for a completelydifferent purpose, is disclosed in Loughman et al patent applicationSer. No. 123,916 entitled "Patient-Operated Pacemaker Programmer", filedon Feb. 22, 1980, which application is hereby incorporated byreference.)

Upon removal of the magnet, flip-flops 12,13 on FIG. 6 are both resetonce again when pin 15 goes high, transistor 8 turns off, and chip IC2is no longer inhibited from generating stimulating pulses. The deviceoperates in the usual way, as described above.

As described above, programming of the device results in the resettingof flip-flops 22-25 and 60-63; each reed closure resets the flip-flopsthrough inverters 26,26A. Thus scanning always begins with theprogrammed values of initial delay and coupled interval since thedecrement-controlling flip-flops are all reset. By monitoring thepatient's ECG waveform and noting the time interval between tachycardiaconfirmation and the generation of a first stimulus, and the timeinterval between the first stimulus and the second, the physician canimmediately determine the programmed values without having to wait forthese values to be reached perhaps ten minutes later during thescanning. Of course, the physician can determine the time parametersquickly only if there is some way to induce tachycardia so that stimuliare generated in the first place. It is also advantageous to allow thephysician to induce tachycardia so that he can observe whether the paceris functioning at all, and also so that he can experiment with differentprogrammed initial delays and coupled intervals to see which are mosteffective in terminating tachycardia.

A mechanism is therefore provided to induce tachycarida which does notrequire any additional components. It will be recalled that the tachyrates which can be programmed by the physician are all within the range130-225 beats per minute, except for the lowest tachy rate of 40 beatsper minute. The tachy rate of 40 beats per minute is not a "real" ratebecause even normal sinus rhythm results in tachycardia confirmation--normal heartbeats occur at a rate greater than 40 beats per minute. Butby allowing such a low rate to be programmed, the physician may possiblyinduce tachycardia.

What happens is that a normal sinus rhythm results in tachycardiaconfirmation and the generation of one or two stimuli. Preferably, atthe same time that the tachy rate is programmed to 40 beats per minute(without changing the initial delay and coupled interval parameters),the pacer should also be programmed to generate a second stimulus alongwith the first. The stimuli soon occur after five normal heartbeats andmay actually induce tachycardia. It has been found that just as one ortwo stimuli shortly after a rapid heart-beat can terminate tachycardia,they can also induce it if the heart was beating in normal sinus rhythm.Once tachycardia is induced, the physician can observe the programmedtime parameters if he so desires; scanning begins with the maximumvalues because the programming itself automatically resets flip-flops22-25 and 60-63 (FIGS. 6 and 8). The physician may then reprogram thepacer to have a tachy rate which is in the "normal" 130-225beat-per-minute range, along with the other parameters (includinginitial delay and coupled interval) whose combined efficacy is to betested. By experimenting in this way, the physician can not only checkthe operation of the pacer, but he can also select optimum parametervalues without the complications of further invasive surgery.

Although the invention has been described with reference to a particularembodiment, it is to be understood that this embodiment is merelyillustrative of the application of the principles of the invention.Numerous modifications may be made therein and other arrangements may bedevised without departing from the spirit and scope of the invention.

What we claim is:
 1. A tachycardia control pacer comprising batterymeans for powering the pacer; means for confirming tachycardia; meansresponsive to said confirming means for generating at least oneheart-stimulating pulse at the end of a time delay following the lastheartbeat within a time range which potentially allows tachycardia to beterminated; externally-controlled means for operating independently ofsaid tachycardia confirming means and for controlling the generation ofat least two pulses separated by a time interval which is dependent uponthe potential of said battery means; said externally-controlled meansincluding a switch operable by a magnet held in the vicinity of thepacer; means for inhibiting operation of said pulse generating means,after said at least two pulses are generated, for as long as said magnetcontinues to be held in the vicinity of the pacer; program control meansfor changing the pacer operation in accordance with externally-generatedsignals which represent programmed parameters, one such change being theselective disablement or enablement of the generation of heartstimulating pulses; said inhibiting means allowing the generation ofpulses to be disabled temporarily until a programmed disablement can beeffected; means for scanning said time delay during successive cycles ofoperation of said pulse generating means; said externally-controlledmeans governing the operation of said program control means to changethe range through which said time delay is scanned in accordance withprogrammed parameters; and means responsive to operation of saidexternally-controlled means for causing the next time delay to bedependent upon only a time delay controlled by said scanning means.
 2. Atachycardia control pacer in accordance with claim 1 further includingmeans under external control for causing the pacer to generate at leastone pulse at a time which potentially induces tachycardia, followingwhich the next time delay represents said time delay programmedparameter.
 3. A tachycardia control pacer comprising battery means forpowering the pacer; means for confirming tachycardia; means responsiveto said confirming means for generating at least one heart-stimulatingpulse at the end of a time delay following the last heartbeat within atime range which potentially allows tachycardia to be terminated;externally-controlled means for operating independently of saidtachycardia confirming means and for controlling the generation of atleast two pulses separated by a time interval which is dependent uponthe potential of said battery means, said externally-controlled meansincluding a switch operable by a magnet held in the vicinity of thepacer; means for inhibiting operation of said pulse generating means,after said at least two pulses are generated, for as long as said magnetcontinues to be held in the vicinity of the pacer; means for scanningsaid time delay during successive cycles of operation of said pulsegenerating means; said externally-controlled means including means forchanging the range through which said time delay is scanned inaccordance with externally-generated signals which represent at leastone programmed parameter; and means responsive to operation of saidexternally-controlled means for causing the next time delay to bedependent upon only a time delay programmed parameter and independent ofthe last time delay controlled by said scanning means.
 4. A tachycardiacontrol pacer in accordance with claim 3 further including means underexternal control for causing the pacer to generate at least one pulse ata time which potentially induces tachycardia, following which the nexttime delay represents said time delay programmed parameter.
 5. Atachycardia control pacer comprising battery means for powering thepacer; means for confirming tachycardia; means responsive to saidconfirming means for generating at least one heart-stimulating pulse atthe end of a time delay following the last heartbeat within a time rangewhich potentially allows tachycardia to be terminated;externally-controlled means for operating indepedently of saidtachycardia confirming means and for controlling the generation of atleast two pulses separated by a time interval which is dependent uponthe potential of said battery means; program control means for changingthe pacer operation in accordance with externally-generated signalswhich represent programmed parameters, one such change being theselective disablement or enablement of the generation ofheart-stimulating pulses; means held operated under external control forcausing the generation of pulses to be disabled temporarily, until aprogrammed disablement can be effected, only for as long as saidexternal control is exercised; means for scanning said time delay duringsuccessive cycles of operation of said pulse generating means; saidprogram control means including means for changing the range throughwhich said time delay is scanned in accordance with externally-generatedsignals which represent at least one programmed parameter; and meansresponsive to operation of said program control means for causing thenext time delay to be dependent upon only a time delay programmedparameter and independent of the last time delay controlled by saidscanning means.
 6. A tachycardia control pacer in accordance with claim5 further including means under external control for causing the pacerto generate at least one pulse at a time which potentially inducestachycardia, following which the next time delay represents said timedelay programmed parameter.
 7. A tachycardia control pacer comprisingbattery means for powering the pacer; means for confirming tachycardia;means reponsive to said confirming means for generating at least oneheart-stimulating pulse at the end of a time delay following the lastheartbeat within a time range which potentially allows tachycardia to beterminated; externally-controlled means for operating independently ofsaid tachycardia confirming means and for controlling the generation ofat least two pulses separated by a time interval which is dependent uponthe potential of said battery means; means for scanning said time delayduring successive cycles of operation of said pulse generating means;program control means for changing the range through which said timedelay is scanned in accordance with externally-generated signals whichrepresent at least one programmed parameter; and means responsive tooperation of said program control means for causing the next time delayto be dependent upon only a time delay programmed parameter andindependent of the last time delay controlled by said scanning means. 8.A tachycardia control pacer in accordance with claim 7 further includingmeans under external control for causing the pacer to generate at leastone pulse at a time which potentially induces tachycardia, followingwhich the next time delay represents said time delay programmedparameter.
 9. A tachycardia control pacer comprising means forconfirming tachycardia; means responsive to said confirming means forgenerating at least one heart-stimulating pulse at the end of a timedelay following the last heartbeat within a time range which potentiallyallows tachycardia to be terminated; externally-controlled means foroperating independently of said tachycardia confirming means and forcontrolling the generation of at least two pulses separated by a timeinterval which is dependent upon a preselected characteristic of thepacer; said externally-controlled means includes a switch operable by amagnet held in the vicinity of the pacer; means for inhibiting operationof said pulse generating means, after said at least two pulses aregenerated, for as long as said magnet continues to be held in thevicinity of the pacer; means for scanning said time delay duringsuccessive cycles of operation of said pulse generating means; saidexternally-controlled means including means for changing the rangethrough which said time delay is scanned in accordance withexternally-generated signals which represent at least one programmedparameter; and means responsive to operation of saidexternally-controlled means for causing the next time delay to bedependent upon only a time delay programmed parameter and independent ofthe last time delay controlled by said scanning means.
 10. A tachycardiacontrol pacer in accordance with claim 9 further including means underexternal control for causing the pacer to generate at least one pulse ata time which potentially induces tachycardia, following which the nexttime delay represents said time delay programmed parameter.
 11. Atachycardia control pacer comprising means for confirming tachycardia;means responsive to said confirming means for generating at least onheart-stimulating pulse at the end of a time delay following the lastheartbeat within a time range which potentially allows tachycardia to beterminated; externally-controlled means for operating independently ofsaid tachycardia confirming means and for controlling the generation ofat least two pulses separated by a time interval which is dependent upona preselected characteristic of the pacer; program control means forchanging the pacer operation in accordance with externally-generatedsignals which represent programmed parameters, one such change being theselective disablement or enablement of the generation ofheart-stimulating pulses; means held operated under external control forcausing the generation of pulses to be disabled temporarily, until aprogrammed disablement can be effected, only for as long as saidexternal control is exercised; means for scanning said time delay duringsuccessive cycles of operation of said pulse generating means; saidprogram control means including means for changing the range throughwhich said time delay is scanned in accordance with externally-generatedsignals which represent at least one programmed parameter; and meansresponsive to operation of said program control means for causing thenext time delay to be dependent upon only a time delay programmedparameter and independent of the last time delay controlled by saidscanning means.
 12. A tachycardia control pacer in accordance with claim11 further including means under external control for causing the pacerto generate at least one pulse at a time which potentially inducestachycardia, following which the next time delay represents said timedelay programmed parameter.
 13. A tachycardia control pacer comprisingmeans for confirming tachycardia; means responsive to said confirmingmeans for generating at least one heart-stimulating pulse at the end ofa time delay following the last heartbeat within a time range whichpotentially allows tachycardia to be terminated; externally-controlledmeans for operating independently of said tachycardia confirming meansand for controlling the generation of at least two pulses separated by atime interval which is dependent upon a preselected characteristic ofthe pacer; means for scanning said time delay during successive cyclesof operation of said pulse generating means; program control means forchanging the range through which said time delay is scanned inaccordance with externally-generated signals which represent at leastone programmed parameter; and means responsive to operation of saidprogram control means for causing the next time delay to be dependentupon only a time delay programmed parameter and independent of the lasttime delay controlled by said scanning means.
 14. A tachycardia controlpacer in accordance with claim 13 further including means under externalcontrol for causing the pacer to generate at least one pulse at a timewhich potentially induces tachycardia, following which the next timedelay represents said time delay programmed parameter.